We worked ******* the implementation of such called shadow fields in Marketo. These fields are not in sync with SFDC.
We created the following fields:
Use this fields in your Form and there will be no sync issues in SFDC.
When a Form is filled out, the MKTO will be changed.
Based on that change we trigger a Smart Campaign in which we create a Task in SFDC - where we show the Shadow Fields and the SFDC Fields for SFDC Contact or SFDC Leads - for the Lead Owner (= SFDC Contact Owner). He can decide to change the SFDC field(s) if necessary.
Changes are then done in SFDC, after synchronization they will be transferred to Marketo.
If you need more specified information, feel free to take contact with me.
Kind regards, Pieter